Eukaryotic protein translation initiation factor 4D (eIF-4D) contains one residue of hypusine and appears to be the only cellular protein with this one unique amino acid. Hypusine is produced post-translationally by transfer of the butylamine portion of the polyamine spermidine to a lysine residue in the eIF-4D precursor and subsequent hydroxylation. These findings reveal a novel cellular metabolic pathway. Comparison of methionyl-puromycin sunthesis activities of mature eIF-4D, eIF-4D precursors that contain unmodified lysine in place of hypusine and a deoxyhypusine-containing derivative of eIF-4D prepared by in vitro modification of cloned eIF-4D precursor by deoxyhypusine synthase indicates that either deoxyhypusine or hypusine is essential for protein synthesis initiation. Mature eIF-4D appear to be vital for growth of eukaryotic cells. Thus, the hypusine biosynthetic step presents a specific potential target for intervention in cellular proliferation. Specific inhibitors of hypusine biosynthesis are being prepared. Studies are underway to relate the structure of hypusine to the physiological function of eIF-4D and to its mode of action in eukaryotic protein synthesis.